As heard on Outdoor Journal Radio
When zebra mussels invade a lake, they bring dramatic changes to the ecosystem. Known for their ability to cover lake bottoms with dense mats, these invasive mollusks disrupt food webs and water quality. Now, a groundbreaking study has revealed an alarming new consequence: zebra mussel infestations are linked to significantly higher mercury levels in fish, particularly walleye and yellow perch.
A Startling Discovery
Researchers from the University of Minnesota, the U.S. Geological Survey, and other agencies conducted a detailed study of 21 lakes in Minnesota. Their goal: to measure mercury concentrations in walleye and yellow perch and determine if zebra mussels play a role.
The results were staggering. Walleye from zebra mussel-infested lakes were found to have 72% higher mercury levels than those from non-infested lakes. Yellow perch, a key prey species for walleye, had mercury levels that were 157% higher. These findings were unexpected and concerning, as both species are popular targets for anglers and vital components of lake ecosystems.
Even smaller and average-sized fish from infested lakes were more likely to exceed the 0.22 parts per million mercury threshold, which triggers stricter advisories for safe human consumption.
How Zebra Mussels Change Mercury Pathways
Zebra mussels impact the environment in ways that inadvertently increase mercury absorption in fish.
Anoxic Sediments:
Zebra mussels create dense mats on lake bottoms, which lead to oxygen-deprived (anoxic) conditions in the sediment. These conditions are ideal for bacteria that convert mercury into methyl mercury, a highly toxic form that fish and humans can absorb.
Nutrient Redistribution:
As filter feeders, zebra mussels extract nutrients from the water and deposit them as waste on the lakebed. This concentrates nutrients near the bottom, forcing walleye and perch to shift their feeding habits closer to shore—precisely where mercury levels are higher.
Where the Study Took Place
The research examined 21 medium-sized lakes in Minnesota, chosen for their suitability as walleye habitats. Twelve of these lakes were infested with zebra mussels, while nine were not. Aside from the presence of zebra mussels, the lakes shared similar environmental characteristics, allowing researchers to isolate the impact of the invasive species.
Popular fishing lakes are especially vulnerable to zebra mussel infestations due to increased boating activity. Hansen pointed out that these are the same locations where the effects on walleye and perch are likely to be most pronounced.
The Minnesota Department of Health regularly issues fish consumption advisories based on mercury levels, with stricter guidelines for children under 15, pregnant individuals, and those who may become pregnant or breastfeed.
